String Theory is our current best candidate for the unification of all fundamental forces, including gravity, in a consistent quantum framework. In this collection of lectures delivered at the Cargese Summer School "String Theory: from Gauge Interactions to Cosmology'', world leading experts provide an up-to-date survey of the latest developments in this topic, including the gauge/gravity correspondence, superstring cosmology and cosmic strings, topological string theory and matrix models, physics beyond the standard model and the landscape of vacua of string theory, conformal field theory and critical phenomena in statistical mechanics. Many more topics are also discussed in shorter contributions by School participants. Written with an emphasis on pedagogy, this volume will be a invaluable resource to students and experts alike.